This invention is in the field of processes for polymerizing fluorinated monomers, in particular aqueous dispersion polymerization processes.
Dispersion processes for polymerizing fluorinated monomers (fluoromonomers) in aqueous media are well known. Such processes employ a surfactant, i.e., a dispersant, to provide stability and permit the polymerization to be carried to commercially acceptable solids concentrations.
Dispersants that have been used in dispersion polymerization processes include the dispersants containing fluoroalkyl groups, such as perfluoroalkyl carboxylic acids and salts, disclosed by Berry in U.S. Pat. No. 2,559,752; the perfluoroalkyl ethane sulfonic acids and salts disclosed by Khan and Morgan in U.S. Pat. No. 4,380,618, Blaise and Grimaud in U.S. Pat. No. 4,025,709 and Baker and Zipfel in U.S. Pat. Nos. 5,688,884 and 5,789,508; the perfluoroalkoxy benzene sulfonic acids and salts disclosed by Morgan in U.S. Pat. No. 4,621,116; the partially-fluorinated carboxylic acids and salts disclosed by Feiring et al. in U.S. Pat. No. 5,763,552; and the perfluoropolyether carboxylic acids and salts disclosed by Garrison in U.S. Pat. No. 3,271,341, Giannetti and Visca in U.S. Pat. No. 4,864,006 and Abusleme and Maccone in European Patent Application Publication 0 625 526. Different dispersants are chosen for use in dispersion polymerization because of their influence on reaction rate, dispersed fluoropolymer particle size, dispersion stability, color and the like. The examples of the ""341 patent reveal that the use of perfluoropolyether carboxylic acids/salt yielded polytetrafluoroethylene dispersions having particle size in the range of 152-299 nm.
Perfluoropolyethers having neutral end groups have been added to dispersion polymerizations as disclosed, for example, in the ""006 patent mentioned above and by Giannetti et al. in U.S. Pat. No. 4,789,717.
Mayer in U.S. Pat. No. 5,563,213 discloses aqueous dispersions of melt-processible dipolymer of tetrafluoroethylene (TFE) and fluoroalkyl perflurovinyl ether having number-average particle size of at most 50 nm. Mayer states that the fluorinated emulsifier is advantageously added in an amount somewhat higher than customary, and the example uses an emulsifier concentration of 0.5 wt % based on the water charge. Mayer indicates that a customary amount is, for example, 0.1-0.15% based on the mass of polymerization liquor. Morgan in PCT Publication WO96/24625 discloses an aqueous process for polymerizing at least two fluoromonomers using a surfactant concentration that is at least 1.2xc3x97 the critical micelle concentration for the surfactant. The disclosed process yields dispersions having small particle size. Example 1 uses a fluorosurfactant concentration of 0.4 wt % based on the water charge (a concentration greater than the critical micelle concentration) to obtain a dispersion of a copolymer of TFE and hexafluoropropylene (HFP) having average particle size of 29 nm.
The ""752 Berry patent cited above also discloses the formation of elongated or ribbon shaped particles of polytetrafluoroethylene (PTFE). Example III for which substantially all of the polymer particles were elongated ribbons used a fluorosurfactant concentration of more than 2 wt % based on the water charge. Berry does not describe the molecular weight of the PTFE resins that he produced. Folda et al. in European Patent Application Publication 0 248 446 disclose a process for producing an anisotropic, liquid crystalline dispersion of TFE polymer by polymerizing in the presence of fluorosurfactant at a concentration in the range extending from the critical micelle concentration to the total solubility of the surfactant. The anisotropic dispersion of Folda et al. contains a high proportion of rod-like particles. The only molecular weight reported by Folda for his rod-shaped resin was 25,000. Seguchi, et al. (J. Polym. Sci., Polymer Phys. Ed., 12, 2567-2576 (1974)) state that higher surfactant levels in emulsion polymerization of PTFE afford rod-like particles but that molecular weight decreases. It is stated that rod-like particles are obtained when the resin molecular weight is between 105 and 5.5xc2x7105 and, furthermore, that granular particles are obtained with molecular weights above 106.
Improved dispersion polymerization processes are desired. Areas for improvement include increased polymerization rate, enhanced incorporation of comonomers having relatively low reactivity, reduced spherical dispersion particle size production of rod-shaped dispersion particles, and reduced coagulum formation, especially with reduced fluorosurfactant concentration.
This invention provides a process comprising polymerizing at least one fluorinated monomer in an aqueous medium containing initiator and dispersing agent to obtain an aqueous dispersion of particles of fluoropolymer, wherein said dispersing agent is a combination of at least two fluorosurfactants, at least one of said fluorosurfactants being perfluoropolyether carboxylic acid or salt thereof, and at least one of said fluorosurfactants being fluoroalkyl carboxylic or sulfonic acid or salt thereof, or fluoroalkoxy aryl sulfonic acid or salt thereof.
The invention further provides a dispersion of substantially-spherical fluoropolymer particles in an aqueous medium containing fluorosurfactant, said dispersion containing at least 20% solids by weight based on total weight of dispersion, said particles having average diameter of no more than 150 nanometer, and wherein the concentration of said fluorosurfactant is no more than 0.35% by weight based on the weight of water in said dispersion.
The invention additionally provides a dispersion of substantially rod-shaped fluoropolymer particles in an aqueous medium containing fluorosurfactant, wherein the concentration of said fluorosurfactant is no more than 0.35% by weight based on the weight of water in said dispersion.
The invention additionally provides a dispersion comprising fluoropolymer particles in an aqueous medium wherein said fluoropolymer particles have a number average molecular weight of at least about 1xc2x7106 preferably at least about 3xc2x7106, and at least about 20% of said particles have a length to diameter ratio of greater than 3.
It has been discovered that aqueous dispersion polymerization of fluoromonomers using a dispersing agent that is a mixture of fluorosurfactants yields improved results when one of the component surfactants present is a perfluoropolyether (PFPE) carboxylic acid or its salt (xe2x80x9cPFPE having carboxyl endsxe2x80x9d). Other surfactant present includes fluorosurfactants commonly used in dispersion polymerization, such as fluoroalkyl carboxylic or sulfonic acid or salt thereof. Improvements include enhanced comonomer incorporation into copolymers, increased polymerization rate, production of rod-shaped particles, and/or reduced dispersion particle size. Surprisingly, the PFPE acid/salt can be a minor part of total fluorosurfactant to achieve such effects, and total fluorosurfactant concentration can be low.
The aqueous dispersion polymerization process of the present invention is conventional except for the use of PFPE carboxylic acid or salt thereof as a component of the dispersing agent for the polymerization of fluorinated monomer. Organic liquid such as 1,1,2-trichloro-1,2,2-trifluoroethane can be present in the aqueous medium, but solvent-free aqueous dispersion polymerization is preferred. The initiator is water-soluble, and will generally be used in the amount of 2-500 ppm based on the weight of water present. Examples of such initiators include ammonium persulfate, potassium persulfate, potassium permanganate/oxalic acid, and disuccinic acid peroxide. The polymerization can be carried out by charging the polymerization reactor with water, surfactant, monomer, and optionally chain transfer agent, agitating the contents of the reactor, and heating the reactor to the desired polymerization temperature, e. g., 25xc2x0-110xc2x0 C., and then adding initiator at the desired rate to start and continue the polymerization. Additional monomer can be added to the reactor to replenish monomer that is consumed. Additional surfactant can also be added to the reactor during the polymerization.
There are several alternatives for regulating the rate of polymerization, and these are applicable for the process of this invention. After initiator injection and reaction kickoff, additional monomer is usually added to maintain the chosen pressure. The monomer may be added at a constant rate, with agitator speed changed as necessary to increase or decrease polymerization rate and thus to maintain constant total pressure. Alternatively, the total pressure and the agitator speed may both be held constant, with monomer added as necessary to maintain the constant pressure. A third alternative is to carry out the polymerization in stages with variable agitator speed, but with steadily increasing monomer feed rates. Another alternative is to hold agitator speed constant and vary the pressure by controlling monomer feed rate to maintain desired reaction rate. One skilled in the art will recognize that other control schemes can be used.
The fluorosurfactant mixture employed in the dispersion polymerization process of the present invention includes fluorosurfactant that contains fluoroalkyl and at most one ether oxygen, and thus is not polyether. If ether oxygen is present, one of the oxygen-carbon bonds preferably links the ether oxygen to a segment of the molecule containing no fluorine. Such surfactants include those commonly used in dispersion polymerization of fluoromonomers. Examples of such surfactants include fluoroalkyl, preferably perfluoroalkyl, carboxylic acids and salts thereof having 6-20 carbon atoms, preferably 6-12 carbon atoms, such as ammonium perfluorooctanoate and ammonium perfluorononanoate (see Berry, U.S. Pat. No. 2,559,752). Additional examples of such surfactants include perfluoroalkyl sulfonic acids and perfluoroalkyl ethane sulfonic acids and salts thereof wherein the surfactant can contain a mixture of perfluoroalkyl having 4-16 carbon atoms and an average of 6-12 carbon atoms (see, Khan and Morgan, U.S. Pat. No. 4,380,618), or can contain perfluoroalkyl having predominantly 6 carbon atoms (see Baker and Zipfel, U.S. Pat. Nos. 5,688,884 and 5,789,508). Additional examples of such surfactants include perfluoroalkoxy benzene sulfonic acids and salts thereof wherein the perfluoroalkyl component of the perfluoroalkoxy has 4-12 carbon atoms, preferably 7-12 carbon atoms (see Morgan, U.S. Pat. No. 4,621,116). Additional examples of such surfactants also include partially-fluorinated surfactants having internal methylene groups and having the formula Rfxe2x80x94(CH2)mxe2x80x94Rxe2x80x2fxe2x80x94COOM wherein m is 1-3, Rf is perfluoroalkyl or perfluoroalkoxy containing 3-8 carbon atoms, Rxe2x80x2f is linear or branched perfluoroalkylene containing 1-4 carbon atoms, and M is NH4, Li, Na, K, or H (see Feiring et al., U.S. Pat. No. 5,763,552). Preferably, the surfactant that is not polyether does not have an ether linkage. Preferred such surfactants include perfluoroalkyl carboxylic acids and salts thereof and perfluoroalkyl ethane sulfonic acids and salts thereof. While more than one such surfactant can be used, normally only one is used.
The perfluoropolyether (PFPE with carboxyl ends) used in this invention can have any chain structure in which oxygen atoms in the backbone of the molecule are separated by saturated fluorocarbon groups having 1-3 carbon atoms. More than one type of fluorocarbon group may be present in the molecule. Representative structures are
(xe2x80x94CFCF3xe2x80x94CF2xe2x80x94Oxe2x80x94)nxe2x80x83xe2x80x83(I)
(xe2x80x94CF2xe2x80x94CF2xe2x80x94CF2xe2x80x94Oxe2x80x94)nxe2x80x83xe2x80x83(II)
(xe2x80x94CF2xe2x80x94CF2Oxe2x80x94)nxe2x80x94(xe2x80x94CF2xe2x80x94Oxe2x80x94)mxe2x80x83xe2x80x83(III)
(xe2x80x94CF2xe2x80x94CFCF3xe2x80x94Oxe2x80x94)nxe2x80x94(xe2x80x94CF2xe2x80x94Oxe2x80x94)mxe2x80x83xe2x80x83(IV)
These structures are discussed by Kasai in J. Appl. Polymer Sci. 57, 797 (1995). As disclosed therein, such PFPE can have a carboxylic acid group or salt thereof (xe2x80x9ccarboxylic groupxe2x80x9d) at one end or at both ends. For monocarboxyl PFPE, the other end of the molecule is usually perfluorinated but may contain a hydrogen or chlorine atom. PFPE having a carboxyl group at one or both ends that can be used in the present invention have at least 2 ether oxygens, more preferably at least 4 ether oxygens, and even more preferably at least 6 ether oxygens. Preferably, at least one of the fluorocarbon groups separating ether oxygens, and more preferably at least two of such fluorocarbon groups, has 2 or 3 carbon atoms. Even more preferably, at least 50% of the fluorocarbon groups separating ether oxygens has 2 or 3 carbon atoms. Also, preferably, the PFPE has a total of at least 9 carbon atoms. While more than one PFPE having a carboxyl group at one or both, ends can be used, normally only one such PFPE is used.
The amount of total fluorosurfactant used in the process of the invention can be within known ranges, which include the customary amount of 0.1-0.15 wt % recited above. Thus, the amount of total surfactant can be from about 0.01 wt % to about 10 wt %, preferably 0.05-3 wt %, more preferably 0.05-035 wt %, based on the amount of water used in the polymerization. The concentration of surfactant(s) that may be employed in the polymerization process of the present invention may be above or below the critical micelle concentration (c.m.c.) of the surfactant. The c.m.c. is different for different surfactants. For example, c.m.c is about 14 g/L for ammonium perfluorooctanoate and 1 g/L for a perfluoroalkyl ethane sulfonic acid such as Zonyl(copyright) TBS or Zonyl(copyright) 6,2-TBS (c.m.c. values determined at room temperature).
As one skilled in the art will recognize, the amount of surfactant required to achieve a given level of dispersion stability will increase with the amount of polymer to be made at constant particle size. The amount of surfactant required for stability also increases with decreasing particle diameter at constant polymer made, since total surface area increases under these conditions. This is observed in some instances for the process of the present invention, which generally yields smaller dispersion particles than a similar process carried out in the absence of PFPE having carboxyl ends. In such instances, if total surfactant is not increased, the resultant dispersion can be unstable at room temperature and form a gel. Surprisingly, resultant dispersions that are unstable at room temperature appear to be stable at elevated temperatures used in polymerization, as judged by the small amount of coagulum in the reactor. xe2x80x9cCoagulumxe2x80x9d is non-water-wettable polymer that can separate from the aqueous dispersion during polymerization. The amount of coagulum formed is an indicator of dispersion stability.
A While PFPE having carboxyl ends can be present in major amount in the dispersant combination, such compounds are costly. Of the total fluorosurfactant, PFPE having carboxyl end groups preferably is present in minor amount, i.e., less than half of total fluorosurfactant by weight. The amount of PFPE having carboxyl ends is more preferably no more than 20 wt %, most preferably no more than 10 wt %, based on weight of total fluorosurfactant. Generally, the amount of PFPE having carboxyl ends present is at least 0.25 wt %, preferably at least 0.5 wt %, based on the weight of total fluorosurfactant. The amount of PFPE having carboxyl endgroups that is used will depend on the level of effect (i.e., the particle size) desired. Surprisingly, the use of PPPE having carboxyl ends alone, e.g., in the absence of fluorosurfactant having at most one ether linkage (not polyether), does not yield improved results compared to the use of fluorosurfactant having at most one ether linkage alone. That is, the use of a combination of at least two fluorosurfactants, at least one of the fluorosurfactants being perfluoropolyether carboxylic acid or salt thereof and at least one of the fluorosurfactants being fluoroalkyl carboxylic or sulfonic acid or salt thereof, or fluoroalkoxy aryl sulfonic acid or salt thereof provides a synergistic effect to the polymerization process, as compared to the use of either type of surfactant alone.
As used herein, xe2x80x9ccombination of fluorosurfactantsxe2x80x9d means that the components of the xe2x80x9ccombinationxe2x80x9d are present in the reactor during polymerization. The components can be introduced separately, including at different times, and need lot be physically combined prior to introduction into the reactor, although they may be so combined. All of the fluorosurfactant may be added to the reactor before polymerization is begun or the addition can be split between a reactor precharge and a later addition, typically after most of the particle nucleation has occurred. The addition of the PFPE is preferably with the precharge.
Fluorinated monomers, i.e., monomers containing at least 35 wt % fluorine, that can be polymerized in the process of this invention include fluoroolefins having 2-10 carbon atoms, fluorinated dioxoles, and fluorinated vinyl ethers of the formula CY2xe2x95x90CYOR or CY2xe2x95x90CYORxe2x80x2OR wherein Y is H or F, and xe2x80x94R, and xe2x80x94Rxe2x80x2xe2x80x94 are independently completely-fluorinated or partially-fluorinated alkyl land alkylene groups containing 1-8 carbon atoms. Preferred xe2x80x94R groups contain 1-4 carbon atoms and are preferably perfluorinated. Preferred xe2x80x94Rxe2x80x2xe2x80x94 groups contain 2-4 carbon atoms and are preferably perfluorinated. Preferred fluoroolefins have 2-6 carbon atoms and include TFE, hexafluoropropylene (HFP), chlorotrifluoroethylene (CTFE), vinyl fluoride, vinylidene fluoride (VF2), trifluoroethylene, hexafluoroisobutylene, and perfluorobutyl ethylene. Preferred cyclic fluorinated monomers include perfluoro-2,2-dimethyl-1,3-dioxole (PDD) and perfluoro-2-methylene-4-methyl-1,3-dioxolane (PMD).
The fluoromonomer may be polymerized alone to form a homopolymer if the fluoromonomer can be homopolymerized, or may be polymerized with one or more other fluoromonomers or other monomers, such as hydrocarbon monomers that are not fluoromonomers, to form a copolymer. If a copolymer is to be formed, the monomers chosen must be able to copolymerize. Fluorine-free monomers that copolymerize with some combinations of fluoromonomers include propylene and ethylene. Examples of useful homopolymers from fluoropolymers include polytetrafluoroethylene (PTFE) and polyvinylidene fluoride. Also usually classed with homopolymer PTFE are the modified PTFE polymers containing fluoromonomers other than TFE in such minor amounts that the modified polymers retain the non-melt-fabricable character of high molecular weight PTFE. Examples of useful copolymers include the copolymers of TFE with HFP and/or perfluoro(alkyl vinyl ethers) such as PPVE or PEVE, copolymers of TFE with PMVE, copolymers of TFE with PDD, and copolymers of TFE or CTFE with ethylene. Further examples include the copolymers of vinylidene fluoride with HFP, or with HFP and TFE. As implied above, copolymers may contain additional monomers beyond those named TFE/ethylene copolymers, for example, are most useful if they include additional monomers that introduce bulky side groups such as PFBE, HFP, PPVE or 2-trifluoromethyl-3,3,3-trifluoro-1-propene, and elastomeric polymers frequently include low concentrations of cure site moieties derived from a cure site monomer.
The polymers of this invention include TFE and CTFE homopolymers; TFE or CTFE polymerized with one or more other fluoromonomers described above such that said fluoromonomers are  less than 1% by weight of the total polymer (wt. %); TFE or CTFE polymerized with 1 to 99 wt. % of one or more other fluoromonomers, preferably 1 to 50 wt. % of one or more other fluoromonomers, more preferably 1 to 20 wt. % of one or more other fluoromonomers, and most preferably 1 to 10 wt. % of one or more other fluoromonomers. In all cases, the wt. % values refer to the amount of comonomer incorporated in the polymer.
Preferred fluoropolymers include the group of tetrafluoroethylene (TFE) polymers. Preferred TFE polymers include perfluoropolymers, particularly TFE homopolymers and copolymers of TFE and one or more of perfluoroolefins having 3-8 carbon atoms, especially HFP, and perfluoro(alkyl vinyl ethers) having alkyl groups containing 1-5 carbon atoms, especially 1-3 carbon atoms.
Fluoropolymers made by the process of this invention can be plastic or elastomeric. They can be amorphous or partially crystalline, melt-fabricable or non-melt-fabricable. As used herein, xe2x80x9cplasticxe2x80x9d has its normal meaning, i.e., that the fluoropolymer will undergo plastic deformation and not recover completely from large deformation. By xe2x80x9celastomericxe2x80x9d is meant that the fluoropolymer is an elastomer or can be cured to be an elastomer as usually defined, i.e., that after being stretched to twice its initial length and released will return to substantially its original length.
Fluoropolymers made by the process of this invention can also contain units derived from monomers that introduce functional groups into the polymer to modify surface characteristics, to provide cross-linking sites, and the like. Functional monomers that introduce pendant side groups having such functionality can have the general formula CY1Y2xe2x95x90CY3xe2x80x94Z wherein each Y is independently H, F or Cl, and Z contains a functional group. Preferably, each Y is F and xe2x80x94Z is xe2x80x94Rfxe2x80x94X, wherein Rf is a fluorinated diradical and X is a functional group that may contain CH2 groups. Examples of such functional monomers include bromotetrafluorobutene. Examples of functional monomers also include fluorovinylethers such as CF2xe2x95x90CF[OCF2CF(CF3)]mxe2x80x94Oxe2x80x94(CF2)nCH2OH as disclosed in U.S. Pat. No. 4,982,009 and the alcoholic ester CF2xe2x95x90CF[OCF2CF(CF3)]m xe2x80x94Oxe2x80x94(CF2)nxe2x80x94(CH2)pxe2x80x94Oxe2x80x94COR as disclosed in U.S. Pat. No. 5,310,838. Additional fluorovinylethers include CF2xe2x95x90CF[OCF2CF(CF3)]mO(CF2)nCOOH and its carboxylic ester CF2xe2x95x90CF[OCF2CF(CF3)]mO(CF2)nCOOR disclosed in U.S. Pat. No. 4,138,426. In these formulae, m=0-3, n=1-4, p=1-2 and R is methyl or ethyl. Preferred such fluorovinylethers include CF2xe2x95x90CFxe2x80x94Oxe2x80x94CF2CF2xe2x80x94SO2F; CF2xe2x95x90CF[OCF2CF(CF3)]O(CF2)2xe2x80x94Y wherein xe2x80x94Y is xe2x80x94SO2F, xe2x80x94CN, or xe2x80x94COOH; and CF2xe2x95x90CF[OCF2CF(CF3)]O(CF2)2xe2x80x94CH2xe2x80x94Z wherein xe2x80x94Z is xe2x80x94OH, xe2x80x94OCN, xe2x80x94Oxe2x80x94(CO)xe2x80x94NH2, or xe2x80x94OP(O)(OH)2. When the purpose is to modify surface characteristics or to provide cross-linking sites, such functional monomers are usually incorporated into the fluoropolymer in minor amount, such as 5 mol % or less, more commonly 3 mol % or less, based on total monomer units in the fluoropolymer. Larger amounts of functional monomer can be incorporated into the polymer for other purposes, e.g., when the copolymer is a precursor to an ion exchange polymer.
The process of the present invention can be used to polymerize tetrafluoroethylene (TFE) to make TFE polymers, i.e., polymers comprising TFE, having high molecular weight or relatively low molecular weight. TFE may be the only monomer used, in which case the polytetrafluoroethylene (PTFE) formed will be homopolymer. Alternatively, an amount of copolymerizable perfluorinated comonomer other than TFE can be added to the reactor to copolymerize with the TFE wherein the resultant high molecular weight TFE polymer is modified with less than 0.5 mol % of the comonomer to impart at least improved film forming properties upon sintering, while still retaining the PTFE character of the polymer (modified PTFE). The PTFE may be non-melt-fabricable, i.e., it will have a melt viscosity (MV) exceeding 1xc2x7108 Paxc2x7s at 380xc2x0 C. MV in this range is measured at 380xc2x0 C. by the tensile creep method described U.S. Pat. No. 3,819,594, the test samples being molded and sintered according to ASTM D-4895. Chain transfer agent, such as ethane or methanol, can be present during the polymerization reaction to provide PTFE having lower MV, e.g., 10 Paxc2x7s to 1xc2x7105 Paxc2x7s measured at 372xc2x0 C. Such Ptfe is commonly known as micropowder, which is described, for example, in ASTM Standard D-5675. Comonomer, if present, will preferably be perfluoro(alkyl vinyl ether), wherein the alkyl group contains 1 to 8 carbon atoms, preferably 1-3 and more preferably 2 or 3 carbon atoms, perhaloolefin such as chlorotrifluoroethylene, perfluoroolefin such as hexafluoropropylene, or perfluoroalkyl olefin such as perfluorobutyl ethylene. More than one modifying comonomer can be used. When TFE is polymerized according to the process of this invention, pressure is typically in the range of 0.3 to 7 MPa and TFE is usually pressured into the reactor at a rate to maintain pressure at a target value. The polymerization is carried out to achieve the desired polymer solids concentration in the aqueous dispersion, e.g. 20 to 60% based on the combined weight of water and polymer solids, and the polymerization is stopped by stopping the TFE feed and venting the reactor to remove unreacted monomer, optionally allowing the reaction to continue for some time after stopping TFE feed and before venting.
When the polymerization process of this invention is used to make a melt-fabricable TFE copolymer, the amount of comonomer added to the reactor will be effective to incorporate sufficient comonomer into the TFE copolymer to reduce the melting point substantially below that of PTFE or modified PTFE, e.g., to 320xc2x0 C. or less, and to make it melt fabricable, which amount will depend on the reactivity of the comonomer relative to TFE and the amount of incorporation necessary to impart melt-fabricability to the copolymer, this too depending on the particular comonomer used. Generally, the amount of comonomer incorporated into a melt-fabricable partially-crystalline TFE copolymer will be at least 0.5 mol % and may be as high as 15 mol % and even higher, depending on the comonomer. The goal of melt fabricability is demonstrable by the copolymer being processible by one or more melt-processing techniques such as extrusion, injection molding, compression molding and the like. Typically, the TFE copoly mer will have an MV in the range of 10 Paxc2x7s to 106 Paxc2x7s . MV is determined by ASTM method D-1238, modified as disclosed in U.S. Pat. No. 4,360,618. The amount of copolymerizable comonomer used will usually be added to the reactor prior to the start of the polymerization reaction, but may also be added during the reaction if desired. One skilled in the art will recognize that a variety of comonomers can be used with TFE to achieve melt-fabricable TFE copolymer, and this variety can be used in the process of the present invention. Examples of copolymerizable perfluorinated monomers include perfluoroolefins such as HFP, or perfluoro(alkyl vinyl ether) (PAVE) wherein the alkyl group contains 1 to 8 carbon atoms, preferably 1-3 carbon atoms. More than one comonomer may be incorporated into the TFE copolymer, which, for example, may be a copolymer of TFE with HFP and one or more PAVE. Representative TFE/HFP copolymers (FEP) and TFE/PAVE copolymers (PFA) are described, for example, in ASTM Standards D-2116 and D-3307.
Other TFE polymers include VF2/HFP/TFE copolymers. As known in the art, such fluoropolymers may be plastic or elastomeric depending on the proportions of monomer types incorporated into the copolymer.
The benefit of the surfactant mixture in the polymerization process of the present invention is evident in the examples in reduced raw dispersion particle size (RDPS) and/or in the formation of rod-shaped dispersion particles. RDPS can be surprisingly small, even smaller than 20 nm, as shown by examples to follow. When the polymer is a TFE polymer and the dispersion particles are predominantly spherical, RDPS can be in the range of from about 5 nm to about 250 nm, preferably 10-200 nm, and more preferably 25-150 nm. Small spherical particles are obtained with copolymers that contain more than about 0.3 mole % (0.5 wt %) comonomer, with TFE homopolymer or copolymers that have a low molecular weight (measurable melt flow). The small spherical particles of low molecular weight PTFE are in contrast to the rod-like particles reported by Seguchi et al. and Folda et al. from the polymerization of low molecular weight PTFE in the presence of high concentrations, i.e. above the c.m.c, of conventional soap. When those batches of high molecular weight TFE particles of our invention that contain rods also contain spherical particles, those spherical particles have reduced diameter. High molecular weight TFE polymers that contain more than about 0.3 mole % comonomer consist of almost exclusively spherical particles of reduced diameter, i.e. less than 150 nm. The amount of dispersing agent used, and the proportion in the total amount of surfactant of PFPE having carboxyl ends, are effective to achieve the dispersion of polymer particles and preferably the preferred particle size within the range recited above. For example, when spherical particles of low molecular weight TFE copolymers are desired, the amount of PFPE having carboxyl ends is about 0.4-20% of the total amount of fluorosurfactant present.
Rod-shaped dispersion particles (length to diameter, or L/D, ratios of greater than 3.0) may be formed if the molecular weight of the TFE polymer is high (not melt fabricable) and the amount of comonomer, if any, is small, that is not more than 0.3 mole %. Dispersion particles with L/D values of greater than 3.0 are sometimes formed during prior art polymerization of high molecular weight TFE polymers but the levels are generally low, about 10 to 15%, and the L/D Values are low, less than 10, usually less than 5, unless the fluorosurfactant level is very high, generally higher than the surfactant c.m.c. value. The addition of PFPE having carboxyl ends to the polymerization allows more rod formation and high L/D values, especially with surfactant levels less than the c.m.c. A reduced amount of PFPE having carboxyl ends can be used with the split addition technique wherein most (more than 50%) of the other fluorosurfactant is added after most of the particle nucleation has occurred (usually after about 10 minutes of polymerization). When high molecular weight TFE polymer with rod-shaped dispersion particles is wanted, the amount of PFPE having carboxyl ends is typically 0.2-10% of the total fluorosurfactant.
Surprisingly, the effects on dispersion particle size and shape can be obtained with the process of the present invention even though the surface tension, as measured at room temperature, of water containing the dispersing agent is not significantly reduced, if at all, by the presence of PFPE having carboxylic acid or salt thereof. For example, the surface tension at room temperature of perfluorohexyl ethane sulfonic acid (6,2-TBS, see Examples below) in water is 26.6 dyne/cm when the 6,2-TBS concentration is 0.094 wt % and 64.4 dyne/cm when the 6,2-TBS concentration is 0.0012 wt %. When a PFPE having carboxylic ends (PFPE-1, see Examples below) is present at concentrations equal to 15% of the 6,2-TBS concentrations, the corresponding surface tensions are 26.5 dyne/cm and 67.8 dyne/cm, respectively. Thus, while the term xe2x80x9cfluorosurfactantxe2x80x9d is applied herein to the PFPE having carboxylic ends, they appear not to be powerful surfactants.
Another embodiment of the present invention is a fluoropolymer aqueous dispersion having substantially-spherical small particles and a low concentration of fluorosurfactant. Fluoropolymers that can be present in such dispersion include the fluoropolymers disclosed above. Preferred such fluoropolymers include either melt-fabricable or non-melt-fabricable TFE copolymers and low molecular weight PTFE (micropowder) as disclosed above. By xe2x80x9csubstantially-sphericalxe2x80x9d is meant than the average ratio of the maximum to the minimum dimensions of particles in an electron micrograph of a dried dispersion specimen is no more than 1.5, using at least 20 particles selected at random to compute the average. By xe2x80x9csmall particle sizexe2x80x9d is meant that the average size off polymer particles, measured as hereinafter described, is no more than 150 n, preferably no more than 75 nm, and more preferably no more than 50 nm. By xe2x80x9clow concentration of fluorosurfactantxe2x80x9d is meant that the total amount of fluorosurfactant present is less than the critical micelle concentration for said fluorosurfactant, preferably no more than 0.35 wt %, more preferably no more than 0.25 wt % and most preferably no more than 0.20 wt %, based on the total weight of water in the dispersion. As recited above, the total amount of fluorosurfactant present is at least 0.01 wt %, preferably at least 0.05 wt %. Thus, the total amount of fluorosurfactant can be in the range of 0.01-0.35 wt %, preferably in the range of 0.05-0.25 wt.%, and more preferably in the range of 0.05-0.20 wt %, based on the weight of water in the dispersion. Surprisingly, such dispersions can have a high content of fluoropolymer solids. The fluoropolymer dispersions of the invention have at least 20 wt % solids based on total weight of dispersion, preferably at least 25 wt % solids. Solids content can be even higher, e.g., 30 wt % or more.
Another embodiment of the present invention is a fluoropolymer aqueous dispersion having particles that are substantially rod-shaped and having a low concentration of fluorosurfactant. Fluoropolymers that can be present in such dispersion include the high molecular weight PTFE and modified PTFE as described above. By xe2x80x9csubstantiallyxe2x80x9d in this context is meant that at least 10%, preferably at least 30%, and more preferably 75%, of the particle volume in an electron micrograph of a dried dispersion specimen are rod-shaped. By xe2x80x9crod shapedxe2x80x9d is meant than the average ratio of the maximum to the minimum dimensions of particles in an electron micrograph of a dried dispersion specimen is at least 3, preferably at least 5, and more preferably at least 10. By xe2x80x9clow concentration of fluorosurfactantxe2x80x9d is meant that the total amount of fluorosurfactant present is less than the critical micelle concentration for said fluorosurfactant, preferably no more than 0.35 wt % and more preferably no more than 0.30 wt % based on the total weight of water in the dispersion. The total amount of PFPE having carboxyl ends present is at least 0.0025 wt % and preferably at least 0.01 wt % based on the total weight of water in the dispersion. The PFPE having carboxyl ends is preferably all precharged to the reactor whereas only a small portion of the other fluorosurfactant is precharged. The remainder of the other fluorosurfactant is preferably added after particle nucleation is largely-complete, typically after 10 minutes of reaction have occurred. This xe2x80x9csplitxe2x80x9d addition of the other fluorosurfactant allows less of the PFPE having carboxyl ends to be used. Surprisingly, such dispersions can have a high content of fluoropolymer solids. The fluoropolymer dispersions of this embodiment of the invention have at least 20 wt % solids based on total weight of dispersion, preferably at least 25 wt % solids. Solids content can be even higher, e.g., 30 wt % or more. Also surprisingly, the resins having a rod-like shape can have a high molecular weight. The molecular weight of these resins can be in excess of 1xc2x7106, preferably in excess of 3xc2x7106.
Fluoropolymers made by the process of this invention can be used in dispersion form for various applications such as metal coating, glass cloth coating, impregnation, and the like. The as-polymerized (raw) dispersion may be used as discharged from the reactor if it has adequate stability and/or wetting characteristics for the intended purpose. Alternatively, the raw dispersion can be adjusted by addition of surfactants, diluted, or concentrated and stabilized by techniques well known in the art. Dispersion concentrations can vary over a broad range, such as from about 10 wt % solids to about 70 wt % solids, based on combined weight of polymer solids and aqueous medium.
Alternatively, the polymer particles produced by the dispersion polymerization process of this invention can be isolated from the aqueous raw dispersion by any convenient means, such as vigorous agitation, optionally supplemented by addition of electrolyte and/or water-immiscible solvent having low surface tension, or by freeze-thaw procedures, following by separation of polymer solids from the liquid and by drying.
The surprising benefit of the surfactant mixture in the polymerization process of the present invention is also evident in the examples to follow in ways that reflect the chosen method of controlling the polymerization. As, outlined above, one method of controlling a polymerization is to vary the intensity of agitation (agitator speed) to regulate mass transfer of gaseous monomer(s) into the aqueous medium to achieve a predetermined rate of polymerization (monomer consumption). Under such a control scheme, all reactions would run at the same rate if the range of agitator speeds is sufficiently broad, and variations in reactivity would be reflected in the speed of agitation necessary to maintain the desired rate, with lower agitator speed indicating higher reactivity. If one chose to run at constant agitator speed, then inherent differences in polymerization rate would be seen directly.
The benefit of the surfactant mixture in the polymerization process of the present invention is also evident in the examples to follow in surprisingly increased comonomer incorporation. For example, in polymerizations to make TFE/HFP copolymer, the amount of HFP incorporated into the copolymer is substantially higher for the process of the invention than for a similar process that does not use the surfactant mixture including PFPE having carboxyl ends. When HFP content is sufficiently high that the TFE/HFP copolymer is amorphous, the process of the present invention permits achievement of high MV more readily than the process of WO96/24625.